Noise at low RPM, goes away at off-idle

[Toro67]

A problem just crept up and it has me stumped. I just finished installing a stereo system and I finished installing my interior. During this time I also upgraded to H4 headlights. I was anxious to get out at night and test the lights out. Everything worked fine and the car was running great. The next afternoon I intended to calibrate my electronic speedometer. I started the car and I hear a rumbling sound from the engine. It’s hard to describe, but it sounds like something came loose and is rattling around hitting something that is rotating. It is very loud so it’s hard to tell exactly where it is coming from, but it sounds like it is coming from the torque converter. My first thought was that the starter did not retract all the way. The weird thing is, my car idles at around 900 rpm in park, if I rev the engine to about 1100 rpm the noise vanishes and the engine runs smooth. I took the car out for a drive and it is shifting well (700R4 trans) and there is seemingly no loss of torque (still bangs through gears and I can light up the tires easily).

So, I check all the fluid levels: all fine. I put the car up on jacks and start it. It still sounds like it is coming from the torque converter. I look at the gap between the flex plate and the starter gear and it’s real close. When the engine is stopped I have about 1/4” gap, but when running that gaps closes up. I have a lot of play in the flex plate. I first check the bolts from the flex plate to the TC, all tight. I can’t check the flex plate to the crank bolting, but the flex plate seems tight. So back to the starter, I shim the motor away from the flex plate a bit more than 1/8”. I was sure this was going to fix it, but it didn’t. So now I am stumped! What can all of a sudden come loose and only make racket at low RPMs, sound like it coming from the TC or at least from underneath the car, and not cause any noticeable performance loss!? Any help will be greatly appreciated.

By the way, the engine is a new crate 350 that I installed this winter. The transmission is also new and was installed last fall.

[HILROD]

The flexplates can crack near the crank flange and cause what you describe. Unbolt the torque converter bolts and push the converter back away from the flexplate, if it rattles or moves at all, it's bad.

[68nate]

What HILROD said, take a close look at the flexplate for cracks. I've seen several crack from the crank mounting hole to the stamped holes in the plate. Makes noise at low engine speed but goes away under load and at higher rpm.

[69muscle]

look for cracked welds where the ring gear meets the flex plate

[Steve68]

I had one fall completely out of the motor after I unbolted the torque converter, car was on a lift, missed my toes by inches, rolled across the floor,

[XCESSIV]

Check torque convertor bolts

[Jim Nilsen]

I had one crack all of the way around at every punched hole in it and it still worked with all of the noises you described. It was a miracle it had even started the car:seizure:

[evan]

Here's an example of a hairline crack in a flexplate.

https://static1.pt-content.com/images/pt/2007/04/gtp034-1.jpg

[Toro67]

found it! I was going to unbolt the torque converter and run the motor uncoupled to rule out the transmission and TC, and that's when I saw a few loose flexplate bolts. I don't know how they came loose, they all have lock washers on them...but...I can remember putting the flex plate on, but I didn't torque the bolts. My friend actually stabbed the motor in. When I got to his shop he was bolting up the trans case...I bet he thought I torqued the flexplate bolts. I'll have to talk to him tomorrow...I bet we both thought the other torqued the bolts. :(

Thanks for the help all!